Up- and down-conversion, and multi-exciton generation for improving solar cells: A reality check

Hagay Shpaisman, Olivia Niitsoo, Igor Lubomirsky, David Cahen

Research output: Contribution to journalConference article

Abstract

Because conventional photovoltaic (PV) cells are threshold systems in terms of optical absorption, "photon management "is an obvious way to improve their performance. Calculations to optimize photon utilization in a single-junction PV cell show -1.4 eV to be the optimal bandgap for terrestrial solar to electrical power conversion. For Si, with a slightly sub-optimal gap, continuous efforts have yielded single-junction laboratory cells, quite close to the theoretical limit. One of the repeatedly proposed directions to improve photon management is that of up-and down-conversion of photon energy. In up-conversion two photons with energy hv < EG (the band gap) create one photon with hv > EG, while in down-conversion one photon with energy hv > 2EG, yields two photons with energy hv > EG. Multi-exciton generation (MEG), although not a "photon management" process, can achieve effects like down-conversion, which, though, is more limited than MEG. In MEG one photon with energy hv > HEG yields n electron-hole pairs with energy EG. Because MEG has clear advantages over down-conversion, in the following we will, instead of considering both, consider MEG. We find that a straightforward analysis of this approach to "photon management" for a single junction cell under the detailed balance limit shows clearly that, even if we assume (highly unrealistic) 100% efficient up-conversion and MEG, a new theoretical PV conversion limit of 49 %, instead of 31% is arrived at, a maximum possible gain of =60%. The main attractive feature of the combination of up-conversion and MEG is a significant broadening of the optimal band-gap range. Rough estimates for the very highest possibly feasible efficiencies for up-conversion and MEG (25% and 70% respectively), yield at most slightly less than 40% PV conversion efficiency, i.e., only a -25% gain over conventional single band gap semiconductor.

Original languageEnglish
Pages (from-to)133-142
Number of pages10
JournalMaterials Research Society Symposium Proceedings
Volume1101
Publication statusPublished - Dec 1 2008
EventLight Management in Photovoltaic Devices-Theory and Practice - San Francisco, CA, United States
Duration: Mar 24 2008Mar 28 2008

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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